Heavy hydrocarbon mixtures contain compounds with high boiling points, and are generally characterized as having high asphaltene content, high viscosity and high density. Today, producers of heavy hydrocarbon mixtures have few options for their use, and the options available have relatively low commercial value.
Asphaltenes are present in heavy hydrocarbon mixtures and have been referred to literally as the “bottom of the barrel” in oil refining. That is asphaltenes are present in heavy hydrocarbon mixtures such as vacuum residues after higher value products, for example, naphtha (for gasoline) and diesel (for diesel fuel), are removed. The heavy hydrocarbon mixtures may further undergo solvent-deasphalting to produce a deasphalted oil (DAO), which can be used, for example, as a feed to a fluid catalytic cracking (FCC) unit.
Some heavy hydrocarbon mixtures are used as residue fuel oil (No. 6 oil), which is a low grade oil, having low value and limited use because of its high viscosity (needs to be heated before use, and cannot be used in today's vehicles) and its relatively high content of contaminants such as sulfur. Heavy hydrocarbon mixtures may be fed to coker units to produce coke. However, coker units are generally inefficient, expensive to operate and susceptible to frequent process upsets and shutdowns, often due to high aromatic content of asphaltenes. Asphaltenes may be used as solid fuels, but sulfur, nitrogen and metal content may be too high to meet quality and emission standards.
Heavy hydrocarbon mixtures may be upgraded through hydroprocessing methods such as hydrotreating and hydrocracking. Large volumes of hydrogen are required for hydroprocessing heavy hydrocarbon mixtures and very large (expensive) reactors are used. High hydrogen uptake that occurs in hydroprocessing heavy hydrocarbon mixtures results in high heat generation, which can result in rapid coking of the catalyst, and catalyst deactivation. High hydrogen input also results in tremendous hydrogen recycle, which requires a high furnace duty (large preheat furnace) and high hydrogen gas compression costs. Furthermore, heavy hydrocarbon mixtures are more likely to experience mass transfer limitations due to their high viscosity (low single pass conversion, need to recycle feed).
Hydroprocessing of mixtures containing relatively high asphaltene content is particularly difficult. Asphaltene-containing mixtures must be heated prior to use to provide a fluid that can be fed to a reactor. However, even when fluid, asphaltenes can form aggregates and clog pipes. Asphaltenes are also known to deactivate catalysts, including by mechanisms in which the asphaltenes form coke, deposits or simply precipitate on the catalyst surfaces. (See, for example, Absi-Halabi, et al., Appl. Catal. 72 (1991) 193-215 and Vogelaar, et al., Catalysis Today, 154 (2010), 256-263.) Therefore traditional options of upgrading feeds having high asphaltene content have been limited.
Still further, removal of nitrogen from asphaltenes is considered difficult. Nitrogen in asphaltenes is mainly contained in heteroaromatic rings, which require a first hydrogenation step prior to removing the nitrogen. Steric effects may further hinder nitrogen removal. (See, Trytten, et al., Ind. Eng. Chem. Res., 29 (1990), 725-730.)
Thus, conventional processes for hydroprocessing heavy hydrocarbons has many disadvantages. It is usually quite expensive (large reactors, large compressors, costs for recycle of both feed and hydrogen, cost to shut down and to replace and/or regenerate deactivated catalyst). There are additional inefficiencies due to recycle of feed because of low conversions. Still further, sulfur, nitrogen, metal and aromatic content present difficulties for some systems.
A number of heavy hydrocarbon mixtures are available from refineries and other sources. Clarified slurry oil (CSO) is a heavy hydrocarbon mixture, which is the bottoms of a fluid catalytic cracking (FCC) unit. CSO represents about 6% of the FCC feed. Heavy hydrocarbon mixtures can also be derived from oil sands. A bitumen-derived heavy gas oil (HGO) can be obtained from oil sands extraction processes. Still other heavy hydrocarbon feeds may be derived from other processes for which higher value uses are desired.
Therefore, there is a need to develop a process for treating heavy hydrocarbon mixtures particularly those having relatively high asphaltene contents, which eliminates above disadvantages, inefficiencies and difficulties with known hydroprocessing processes. The present invention provides a process to upgrade heavy hydrocarbon mixtures and thus increase the value that can be derived from such mixtures.